<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>控制样式</title>

    <style>
      .d1{
        border: 1px solid red;
        width: 100px;
        height: 100px;
      }
      .d2{
        border: 1px solid blue;
        width: 200px;
        height: 200px;
      }

    </style>
</head>
<body>

    <div id="div1">
      div
    </div>

    <div id="div2">
      div
    </div>

    <script>
      var div1 = document.getElementById("div1");
      div1.onclick = function () {
        //修改样式方式1
        div1.style.border = "1px solid red";

        div1.style.width = "200px";

        //font-size --> fontSize
        div1.style.fontSize = "20px";
      }

      var div2 = document.getElementById("div2");
      div2.onclick = function (){
        div2.className = "d1";
      }

    </script>

</body>
</html>